An optimal HIV vaccination strategy would elicit persistent, sterilizing, cross-clade neutralizing antibodies against the virus. This goal will likely require the development of novel antigen delivery strategies in which viral vectors play a major role. Improvements in the construction of vectors based on the linear, single-stranded DNA genomes of the autonomously replicating parvoviruses have recently allowed us to produce vectors, in amounts suitable for use in vivo, devoid of replication competent virus. These vectors are predicted to share several unique properties with their parental viruses. They should be unable to initiate replication in resting cells, and persist cryptically until the host cell enters S phase in response to exogenous signals, at which time they would replicate, express the transgene they encode, and kill the cell. Additionally, we should be able to target them to express in particular cell types as a property of the coat in which they are packaged. We will use vectors packaged in the coat of the murine lymphotropic parvovirus MVMi to target foreign antigen expression to a very small fraction of T-cells in the host. Since most of these infected cells will be resting, very little antigen should be expressed at any one time. However, since these cells will likely be activated for proliferation stochastically at some point in the life of the host, they will become a continual source of infected, dying cells expressing antigen, thus eliciting a persistent immune response to encoded antigen. Preliminary results show that mice infected with an MVM vector expressing the Borellia surface antigen OspA mount a vigorous, protective humoral response which is still maximal at 150 days after a single intravenous inoculation with as few as 105 transducing units of vector. We will use vectors encoding green fluorescent protein to measure the ability of the vector to infect resting murine, human and macaque T-cells, and express them conditionally upon activation. A further vector construct expressing a synthetic chimeric antigen will be used to monitor several aspects of this unique virus: host interaction, and ELISA assays developed to analyze the biology and longevity of the immune response induced. The effect on induced immunity of mutation of both non-structural and capsid genes of the vector will be examined. C-terminally truncated versions of HIV-1 gp160; with or without glycosylation modifications will be constructed into the vector. Antibody responses induced by these vectors, initially in mice, will be quantitated by ELISA and analyzed for neutralizing activity, firstly with a "surrogate" gp160-carrying VSV vector and then in cross-clade HIV neutralization assays.